Because Bhaichung was in Kolkata. Chettri though being an inspirational icon he is, didn't play much here and Bengaluru isn't Kolkata. Kolkata is the Mecca of Indian football and if you don't play here, we will admire you but will never really love you. So if I am a sports oriented brand, I don't quite see why should I have a Chettri as my face.
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Article is from 2nd December, stadium looks in the same condition today also
The league was supposedly awaiting the completion of Chennai City's new venue before finalising its fixtures, but weeks later its still not ready.
https://khelnow.com/news/article/i-league-2017-18-schedule-delay-chennai-city-coimbatore-stadium-exclusive